Add plugin template for user documentation

[MartinHjelmare]

Roadmap: CI, contribution guidelines, project management

We want to make it easy for plugin developers to get started and reach a high standard for their plugins. Do this by creating templates for plugins.

• Add a template for plugin user documentation.
• Include guidelines for "get started" documentation for different plugin types.
• Include these things in the template:
  • Pluging name
  • Description
  • How to use the plugin
  • How to cite the plugin author
  • Contributor instructions